Part of the process of refining crude oil into usable hydrocarbons involves separation of the denser materials from the lighter liquid hydrocarbons. The liquid hydrocarbons removed from the denser materials are further refined into gasoline and chemicals used for a variety of purposes in industry. The refining process involves heating the liquid hydrocarbons in successive steps to a temperature in which the desired hydrocarbon is vaporized. The vaporized hydrocarbon can be removed from the non-vaporized materials and collected in a separate vessel. Cooling of the vaporized hydrocarbon causes the vaporized hydrocarbon to return to the liquid form. Each hydrocarbon has a specific temperature at which it becomes a vapor. By heating the hydrocarbon-containing materials to a specific temperature, a specific product may be isolated and collected. Heating and reheating these hydrocarbon-containing materials to various temperatures eventually results in removal and collection of the valuable hydrocarbons which can then be used for a variety of purposes. Heating of the liquid hydrocarbon initially occurs in a coker heater. U.S. Pat. No. 5,804,038 and Patent Publication No. US 2002/0157987 A1 disclose exemplary coking systems and associated equipment including a coker heater or furnace. U.S. Pat. No. 5,804,038 and Patent Publication No. US 2002/0157987 A1 are incorporated herein by reference in their entirety for all purposes.
Coker heaters have been used to heat a fluid, such as a heavy cut of liquid hydrocarbons or crude oil, to temperatures approximately 920 degrees Fahrenheit (493 degrees centigrade) to facilitate thermal cracking and solid coke formation in the petroleum refining industry. These coke heaters are positioned in coke drum vessels used in the petroleum coking process. In the coking process, a layer of solid coke petroleum refining industry. These coke heaters are positioned in coke drum vessels used in the petroleum coking process. In the coking process, a layer of solid coke forms on the inside surface of the pipes or tubes positioned in a radiant section of the heater. The heater radiant section is where heat is transferred from a plurality of heater burners to the liquid hydrocarbons.
In some coker heaters two to four pipes are positioned in a horizontal orientation in the heater radiant section for passing or flowing liquid hydrocarbons. The horizontal pipes are heated by the burners so that the liquid hydrocarbons are heated in the pipes to about 920 degrees Fahrenheit (493 degrees centigrade). During this heating, coke is removed from the liquid hydrocarbons. Some of the coke that is removed from the liquid hydrocarbons is deposited on the inside of the pipes. Periodically, the deposited coke in the pipes must be cleaned out to restore the flow capacity of the pipes.
Cleaning of the pipes can be performed by one of two methods or both of the methods in combination. The first method of cleaning the deposited coke out of the pipes is called spalling. The second method of cleaning the deposited coke out of the pipes involves moving a mechanical pig through the pipes to mechanically scrape or remove the coke from the inside of each of the pipes. Spalling involves taking a coke-coated on-line pipe out-of-service so that it can cool. The pipes cool from about 1290 degrees Fahrenheit (700 degrees centigrade) to about 700 degrees Fahrenheit (371 degrees centigrade). During the cooling, some of the coke deposited on the inside of the on-line pipe breaks free or flakes off as the out-of-service pipe shrinks in size during cooling. The loose coke is then flushed out of the pipe, and collected in a tank, using boiler water or steam. The collected coke may be used as a fuel in other processes, as a hardener in the metallurgy industry or further fractionated to collect other valuable hydrocarbons. A typical spalling of a pipe takes about two days. During the time period when the on-line pipe is out-of-service no liquid hydrocarbons pass through the pipe so the overall flow capacity of the coker is reduced. For example, in a coker with four pipes passing through the heater radiant section, a throughput of about 10,000 barrels/day of liquid hydrocarbons through each pipe can be expected when the coker begins operation for a total design charge rate of 40,000 barrels/day. If one pipe is taken off-line or is out-of-service for spalling for two days, there is a 20,000 barrel loss of throughput for the two days. Depending on the chemical characteristics of the crude oil processed by the coker heater, spalling of the pipes in the coker heater occur every two to nine months.
Because spalling does not completely clean out the pipes running through a coker, many refinery operators use a mechanical pig to clean out all of the pipes about every eight to ten months. In “pigging”, a foam or plastic pig with metal studs and grit could be passed through the on-line pipe. As it is passed through the pipe the pig rotates and scrapes the coke off of the inside of the pipe. During the process “pigs” of different sizes and abrasiveness can be used to remove most all of the coke on the inside of the pipe. Typically, the mechanical pigging takes about five days. During this five-day period, there is no throughput of liquid hydrocarbons through the coker. Further, “pigging” is usually performed by an outside vendor resulting in additional cost to the refinery operator.
Thus, because of the need to remove deposited coke from the pipes, a refinery operator loses the profit that can be made processing liquid hydrocarbons each time the two day spalling is performed in addition to the loss of profit when the coker is completely off-line for the five day mechanical pigging.
Those of ordinary skill in chemical process plant engineering also understand that the refinery operator suffers other losses from the coke deposited on the inside of the pipes. Specifically, if all of the coke inside the pipes is not removed by spalling, the coke which remains inside of the pipes after spalling restricts the size of the opening through which the liquid hydrocarbons may pass thereby reducing the throughput of liquid hydrocarbons.
Additional losses occur as the coke deposited on the inside of the pipes acts as a heat insulator. The outer skin temperature of a clean pipe passing through the heater radiant section may only need to be 930 degrees Fahrenheit (510 degrees centigrade) to heat the liquid hydrocarbons to 920 degrees Fahrenheit (493 degrees centigrade). However, as the coke builds up on the inside of the pipe, the skin temperature needed on the outside of the pipe to heat the liquid hydrocarbons in the pipe to 920 degrees Fahrenheit (493 degrees centigrade) may increase the needed pipe skin temperature to be as much as 1250 degrees Fahrenheit (677 degrees centigrade). Two consequences are associated with higher skin temperatures on the outside of the pipe. First, more energy is needed to achieve these higher pipe skin temperatures and, second, the service life of the pipe is decreased when it must be maintained at higher temperatures for longer periods of time. The increased energy and the decreased life span of the pipe increases the cost to the refinery operator for refining liquid hydrocarbons. Inevitably, that cost must be passed along to the consumer.
Accordingly, it would be desirable to provide an on-line spalling system and method that reduces the cost of operating a coker.